As far as durability goes, I went caseless on all iPhones except 5 and 5s... Just worried ill scratch the aluminum back 
The 3G, 3GS design kind of sucked. So when the 4 came out with its sleek boxy, stainless steel band, dual glass faces it was a game changer.
I don't see iPhone 5c on your poll.
You missed the 5c off that list.
And that's where my vote would have gone.
As far as durability goes, I went caseless on all iPhones except 5 and 5s... Just worried ill scratch the aluminum back![]()
The original, the 4, the 5s...
In that order.
I've owned every model. The original and the 4 were radical.